[SOLVED] O-scope Voltage Percentage Measurement Cursors to measure stage Gain

[danny davis]

At work they have tektronix O-scopes 2212, 2221, 2445

The O-scopes have a Percentage for measure voltage percentage , to measure the voltage percentage of 2 different voltages

Can I use this to measure the input signal of a op amp on ch1 and the output signal on ch2

The Percentage would tell me the voltage difference percentage between the two signals from input and output of that stage

My question is , one I got the percentage from the O-scope reading , is this the GAIN? or how can i calculate the gain from this percentage?

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It's a voltage ratio between the input signal and the output signal

The O-scope will display the voltage RATIO

This Voltage ratio is the GAIN of that stage?

 

[Audioguru]

You forgot to translate the word "gain" into your own language. English dictionaries define gain as "the ratio of the signal output of a system to the signal input of the same system". Why didn't you look it up?

We think you want to measure the voltage gain but instead maybe the current gain or the power gain?

 

[danny davis]

Yes it's the Voltage Ratio, on the O-scope you're measuring the Voltage Ratio , it has cursors , the O scope will display the Voltage ratio

Isn't this the voltage gain of the op amp state? or transistor stage?

I know Audio Mixers have multiple Rin resistors, This O-scope feature of measuring the Voltage RATIO, can measure the RATIOS of each Rin input compared with the output of the stage?

 

[Audioguru]

I have made thousands of amplifiers with transistors and opamps and I HAVE NEVER measured the voltage gain because it is ALWAYS correct. The voltage gain is determined by the ratio of 2 resistors. Why are you trying to measure voltage gain and why use a 'scope to do it?

If you want to know the voltage gain then simply look at the value of the two resistors that determine the gain.

 

[danny davis]

To measure the voltage RATIOs on the O-scope, it's used for measuring mixers , mixing different waveforms together

So you know the voltage ratio for each Rin compared to the Voutput for each waveform that you're mixing

What would you use the O-scope to measure voltage ratios for?

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At work , there is summing amps or summing stages , summing circuits that merge together or summing together different waveforms or DC voltages

The O-scope voltage Ratio percentage measurement using cursors is good for measuring summing amps, stages, circuits?

Each summing amp have 2 or 3 inputs that sums together , but how do you know the Voltage Ratio for the summing?

Using the O-scope voltage ratio cursors you can measure the summing input ratios? and output ratio of the summed waveforms?

 

[Audioguru]

Why are you measuring the voltage ratios of a mixer or a summing amplifier? The voltage ratios are simply determined by the resistor ratios.
Has anybody ever seen a mixer or summer with an error in the voltage ratios? I haven't.

 

[danny davis]

Why are you measuring the voltage ratios of a mixer or a summing amplifier?

Yes I am

The voltage ratios are simply determined by the resistor ratios.

yes I know

But the O-scope voltage ratio cursors can measure each Rin waveform to measure the summing ratios for each input to output

 

[Audioguru]

Are you getting paid to waste time like that at work?

 

[danny davis]

I'm troubleshooting summing sections, stages

It's ment for troubleshooting , can't u see it helps

 

[Audioguru]

What is wrong at the output of the summing stage? Post the schematic and show what it is supposed to be and what is the error.

 

[danny davis]

What is wrong at the output of the summing stage?

The voltage ratios are off , or it's not summing right , from the resistance drifting out of tolerance or the op amp is defective or the GAIN's are off

 

[Audioguru]

Why didn't you post the schematic of a summing stage?
Do you see resistor values that have drifted or resistors that have the wrong value?
Have you seen opamps without enough gain?

 

[danny davis]

No the Rin resistors drift in value up and down, this causes the summing of the voltage ratios to be not equal and off

or it is caused by stages before the Rin , that causes the signal before the Rin to have less voltage , so it has less gain and less output voltage, so the summing of the waveforms ratio is OFF balanced

It's not that cut and dry when troubleshooting summing amps that has Signal gain or voltage problems

 

[Audioguru]

I saw the a resistor with a wrong value about 49 years ago. It was a carbon composition type. Modern carbon film or metal film resistors do not drift unless a stupid "technician" overheats or suddenly freezes them trying to find a bad one. The sudden temperature change fractures them internally and changes their value or makes them intermittent.

Do the circuits you work on have resistors that are 49 years old? In another thread you said you overheated and suddenly froze resistors and ICs trying to find (cause?) a bad one.

 

[danny davis]

They are carbon Composition type resistors in the all the circuits at my job, so the summing amps are carbon comps

What would you use the O-scopes voltage RATIO cursors to measure? it's for measuring voltage ratios , for what kind of circuits or stages?

I'm using it to troubleshoot summing amps, stages

Don't you think it's good use to that?

 

[betwixt]

I think we are avoiding (evading?) the original question:

NO, do not use the ratio cursors if the scope has them. I assume you mean % markers on the graticule, if so they are used for something else altogether.
The way to do it to use one probe, preferably a 'x10' type which has been properly compensated. Measure the amplitude of the waveform using one of these methods:

1. Make sure you know where the zero line is by switching the Y input to ground and adjusting the Y position to overlay one of the horizontal lines on the graticule.
2. Switch the Y input to DC coupling. Now measure the voltage as the number of squares the line deflected multiplied by the V/division on the Y amplitude control.
3. Do steps 1 and 2 for the input of the summing network and the output of it. Use a calculator to work out "measured output / measured input", this gives the gain.

or if you want to use the scope 'on screen' cursors:

1. set the scope to show the difference between cursor voltages.
2. set one cursor on the bottom of the input signal, the other to the top of the signal, if you are measuring DC, set the bottom cursor to show 0V. Note the top cursor reading.
3. repeat step 2 on the output signal, again note the top cursor reading.
4. Use a calculator to work out "measured output / measured input", this gives the gain.

If your summing amplifiers have steady DC inputs, it is easier and probably more accurate to use a DVM to measure the voltages. In any case, if you are fault finding there probably isn't any need to check gain at all. If the resistor values measure OK the stage is almost certainly working properly.

Brian.

 

[danny davis]

NO, do not use the ratio cursors if the scope has them

What do you use the Voltage RaTio cursors for ? to measure what? and for what circuits or stages?

 

[betwixt]

First tell me what you mean by the "ratio" cursors on your scope. There is often a marker of the graticules for measuring percentage rise and fall times but these are not normally related to the cursors.

Brian.

 

[danny davis]

tektronix O-scopes 2445 Manual
GO to 6-2 its Voltage RATIO chapter , shows you how to measure voltage ratios using the ratio cursors on the O-scope
Look at the picture figure 6-2 and it shows you
**broken link removed**

 

[Audioguru]

What circuit do you have that needs its voltage ratios measured? Please post its schematic.
Do you know what the ratios are supposed to be?
Why did you suspect that the voltage ratios are wrong?
Do you know what causes the voltage ratios to be wrong?